1. Field of the Invention
The present invention relates to a device for printing or copying in which least one toner mark on a toner carrier is inked with toner for monitoring and adjusting the toner surface coverage. The invention also relates to a method for printing or copying.
2. Description of the Related Art
A conventional printer or copier has a toner carrier, for instance a photoconductive drum, on which a latent image is generated, for instance by exposure of the drum with light. A developing station serves for inking the latent image with toner. This developer station contains a developer mixture of toner and a carrier, for instance magnetic iron particles, with an adjustable proportion of toner. In order to be able to check the toner surface coverage and adjust it, a toner mark is made on the toner carrier and is scanned, for instance using a reflex sensor.
The toner surface coverage in such a two-component developing system is essentially determined by the following factors:
a) the toner supply in the developer zone; i.e. at the contact surface of developer station and the surface of the toner carrier;
b) the toner concentration in the developer mixture;
c) the electrostatic charge behavior of the surface of the photoconductive drum;
d) the trieboelectric behavior of the developer mixture; that is, the force of the adhesion between carrier particles and toner;
e) the shape and size of the particles in the developer mixture;
f) the activity of the toner particles which are provided by the developer drum in the developer zone.
For a specific type of device, the factors c to f are relatively constant parameters. With respect to factors a and b, there is a mutual dependency; namely, the toner supply in the developer zone is dependent on the toner concentration, so that the contrast level of the latent image is determined by the toner concentration. This contrast level or the toner surface coverage area is proportional to the toner concentration.
In previous printers or copiers, the toner surface coverage has been adjusted by measuring the toner marks, for instance using a reflex sensor. The signals of the reflex sensor has then served as a measure of surface coverage; that is, the darker the inking of the toner marks with toner, the lower the signal level of the voltage of the receiver which picks up the reflected radiation. But this signal level is also dependent on the reflection behavior of the toner and of the character of the surface of the toner carrier, for instance of the photoconductor. Furthermore, the tolerances of the reflection sensor which scans the toner marks must be taken into account. It is therefore state of the art to perform an individual adjustment for each printer of the developer station, the toner carrier, and so on, to the toner material. When the toner type is changed and the toner carrier is replaced, this adjustment must always be performed again. In conventional printers, correction programs were installed to facilitate the adjustment work, which performed an automatic adjustment within particular bandwidths once the toner type and the type of photoconductive drum were inputted. But the result is frequently unsatisfactory despite these corrective measures, and a toner excess can set in, in which the toner layer on the toner carrier is thicker than necessary, which leads to excessive consumption of toner.
German Patent Document DE-A-39 38 354 teaches an image recording device in which a toner density sensor tests two toner marks on a photoconductive drum. The sensor contains two photoreceivers, which evaluate the degree of reflection of the two toner marks. The toner marks are arranged transverse to the direction of motion of the photoconductive drums, with one toner mark having a high density and the other toner mark, a low density. A bias voltage for the developer unit is set depending on the measurement result of the photoreceiver.
U.S. Pat. No. 5,410,388 teaches a device, or respectively, a method for electrographic printing or copying in which a densitometer determines the respective surface coverage at two consecutive measurement locations of an elongated toner mark using infrared radiation. When there is a difference between the surface coverage in the front region of the toner mark and the back region of the toner mark as expressed in a different degree of reflection, parameters of the development process are readjusted; for example, the toner concentration can be modified. If the difference in the reflection power or in the values of the surface coverage equals zero, then the developer process is left unchanged.
It is an object of the invention to provide a device and a method which reduce the adjustment outlay and which achieve a relatively high-quality print result.
This object and others are achieved for devices by claims 1, 13, 22, 30 and Sub.
The invention makes use of an effect that occurs in the inking of a solid surface with toner in the direction of motion of the toner carrier in two-component developer mixtures. As viewed in the direction of motion of the toner carrier, the toner supply within the developer zone changes. At first the toner supply is large, which leads to a high surface coverage. When this first supply of toner has been transferred to the toner carrier, then new toner must first be transported by the developer drums to replace it, which leads to a reduction of surface coverage given low the toner concentration. After the initial drop in toner supply, the supply of toner as seen across the length of the toner mark remains constant, and thus a constant surface coverage also sets in. In this regard, one refers to a depletion effect in the solid surface in the direction of motion of the toner carrier. This depletion effect manifests itself in a reduction of the surface coverage, until saturation is achieved; that is, within the depletion zone the surface coverage is 100% on the toner mark. Given this degree of surface coverage, the toner mark is covered with toner densely without gaps; given a black toner, increasing the layer thickness of the toner does not produce any additional blackening. The fluctuations in surface coverage along the toner mark are inversely proportional to the toner concentration. The higher the toner concentration, the smaller the differences in surface coverage, or respectively, contrast level on the toner marks.
The toner marks that have been inked with toner are now inventively scanned by at least one sensor at at least two consecutive measurement points as seen in the direction of the motion of the toner carrier, and the surface coverage at these measurement points is imaged as electrical signals. The proportion of toner in the developer mixture is adjusted dependent on the difference or the quotient of the absolute values of the signals at these two measurement points. Thus, it is not the absolute level of the signal of the sensor which is evaluated, but rather the difference in the signals, or respectively, the quotient of the signals, that is measured along the toner marks. This difference, or respectively, the quotient, is largely independent of the reflection power of the utilized toner, and it is therefore unnecessary to perform different adjustments for different types of toner. The reflection power of the surface of the toner carrierxe2x80x94for instance of the surface of a photoconductive drum, or that of a carrier material made of paperxe2x80x94on which the toner mark is printed and then scanned enters into the result only slightly, in particular when calibration is performed to the reflection behavior of the respective surface, as described below.
When the proportion of toner in the developer mixture is adjusted in such a way that the difference is close to zero or the quotient is close to one, then almost no fluctuation of the surface coverage occurs over the length of the toner mark. In this operative state, an optimal contrast level is guaranteed without a toner excess.
One exemplifying embodiment is characterized in that the difference or the quotient of the signals at the two measurement locations is compared to a desired value, and that, depending on the comparison, a controller actuates a conveyor mechanism that transports toner to the developer station. This creates a control system which ensures that the printer is always held in an optimal operating state with high-quality print results. A value close to zero is selected as desired value for a difference evaluation, and a value close to one is selected for a quotient evaluation. When the control process commences given a certain toner deficitxe2x80x94that is, the difference is greater than zero or the quotient is not equal to onexe2x80x94then the subsequent control process ensures that an operating state with a toner surplus does not set in, since a certain deviation to the desired value remains. A time-point controller is preferably used as the controller, which switches the conveyor mechanism back and forth between the ON and OFF states.
In the above described device and evaluation method, it is essential that the relative position of the measurement locations on the toner mark remain the same. Mechanical assembly tolerances of the toner carrier or the sensor can lead to changes in the distance between the toner carrier and sensor. Such position changes can also occur when the toner carrier or the sensor is replaced. When the toner mark is now scanned by the sensor following a predetermined delay period in which the toner mark that has been inked with toner is moved forward subsequent to the writing of the latent images until it reaches the sensor, not always the same locations of the toner mark are measured. As a result, the setting obtained from the electrical measurement signals of the sensors is no longer optimal. Accordingly, INSERT CLAIMS 22 and 23 a device and a method are provide, which permit scanning of the toner mark at measurement locations whose position relative to the toner mark is constant.
In accordance with this aspect of the invention, the reference time at which a reference point on the toner mark passes the scanning sensor is fixed by a stationary scanning sensor, which is preferably the same sensor that scans the toner mark at the two measurement locations. The front edge or back edge of the toner mark is preferably used as a reference point. The additional times at which the two measurement locations are scanned can be specified dependent on this reference time given knowledge of the transport speed of the toner carrier. These measurement locations are then scanned in each toner mark with respect to the reference point at defined intervals from this reference point.
The described part of the invention is preferably used when the evaluation of the electrical signals occurs in accordance with the above described device and method. But it can also be used advantageously to fix the position of a toner mark that is scanned at two measurement locations.